Speed reduction gears are used for electric power steering apparatuses for automobiles. In a column type EPS (Electric Power Steering System), for example, rotation of an electric motor is transmitted from a small gear such as a worm to a large gear such as a worm wheel in a speed reduction gear to reduce its speed as well as to amplify its output, and is then provided to a column, thereby torque-assisting a steering operation.
Suitable backlash is required to engage the small and large gears which serve as a speed reduction mechanism. At the time of forward/backward rotation of the gears and in a case where an automobile travels on a punishing road such as a stone pavement so that a reaction force from a tire is inputted, for example, however, a teeth striking sound may, in some cases, be produced due to the backlash. When the teeth striking sound is transmitted as noise to the inside of the automobile, an uncomfortable feeling is given to a driver.
Therefore, so-called by-layer assembly for selecting a combination of the gears such that suitable backlash is obtained to assemble a speed reduction gear has been conventionally carried out. In such a method, however, the productivity is significantly low. Another problem is that even if the by-layer assembly is carried out, there occurs nonuniformity of a steering torque due to offcenter of a shaft of a worm wheel.
The same problems exist in not only speed reduction gear in electric power steering apparatus but also general speed reduction gear having the small and large gears.
In a speed reduction gear in an electric power steering apparatus, for example, therefore, it has been proposed that backlash is eliminated by providing urging means such as a spring member for making it possible for a worm shaft to be eccentric toward a worm wheel as well as urging the worm shaft in the direction of eccentricity (see Japanese Unexamined Patent Publication No. 2000-43739, for example).
However, the configuration of the above-mentioned speed reduction gear becomes significantly complicated, resulting in raised manufacturing cost.